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VALVES + ACTUATORS
W
hen assessing control valves and their performance, you
need to understand the different types of valves and what
they can be tested for. For open/close shutoff valves with-
out analog control the tests are pretty simple.
Do the valves open and close? When open, do they open all the
way? When they close, do they close completely? Testing is mostly
observational – looking at the valve and watching cause and effect
in the process during the cycle.
Control valves are a different ‘beast’ altogether
These valves open and close proportionally, and vary the degree of
travel depending on the percent of span of the 4 to 20 mA signal ap-
plied to them. Observing the valve’s position, as reported on the visual
Jim Shields, Fluke Corporation
What are the types, what can they be tested for?
Assessing
control valves
and their
performance
travel indicator, gives the technician or operator a rough indication
of percent of travel for a particular setting when in operation - but
does not provide any assurance of how the valve will operate under
dynamic and changing conditions.
The most sophisticated valve performance tests require removing
the valve and testing its performance on a ‘valve prover.’ This is an
expensive test device, out of range for most instrument shops. The
valve prover is often only used by valve manufacturers in testing the
valve when shipped, or by highly qualified field service engineers.
It offers a very complete test, but the tool is not feasible for most
instrument shops.
So, what is the technician to use for testing a control valve? What
is a meaningful test that can be used as a baseline? Since most valves
use a 4 to 20 mA input signal, any test tool with an mA output signal
Electricity+Control
July ‘16
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